The Third Line — Lymphocytes
While inflammation and phagocytes fight every invader the same way, your third line of defence is different. It learns, remembers, and targets specific enemies with precision. B cells, T cells, and antibodies form the most sophisticated defence system in the known universe — and it lives inside you.
Before You Begin
Imagine you catch a disease, recover, and then encounter the same disease years later.
Write down your answers before reading on:
- Why do you usually not get sick the second time?
- What do you think your body remembers about the first infection?
- Why do some diseases (like chickenpox) give lifelong immunity while others (like flu) do not?
Know
- The components of the third line of defence: B cells, T cells, antibodies
- How antigens trigger the specific immune response
- The difference between the primary and secondary immune response
Understand
- Why the third line is called "specific"
- How memory cells enable faster future responses
- The roles of B cells and T cells in immunity
Can Do
- Explain how antibodies recognise and neutralise specific pathogens
- Describe the primary and secondary immune responses
- Compare the second and third lines of defence
Antigens and Specificity
How the immune system recognises enemies
Inflammation
Every pathogen has unique molecules on its surface called antigens. Think of antigens as identity badges that tell the immune system exactly what it is facing.
The third line of defence is specific because lymphocytes produce responses tailored to particular antigens. Unlike the second line (which attacks all pathogens the same way), the third line creates customised weapons for each enemy.
When a pathogen enters the body, its antigens are detected by specialised cells called lymphocytes. There are two main types:
- B cells — produce antibodies that bind to antigens
- T cells — kill infected cells and coordinate the immune response
Each lymphocyte is programmed to recognise one specific antigen. Your body contains millions of different lymphocytes, each waiting for its matching antigen.
B Cells and Antibodies
The molecular weapons
When a B cell encounters its matching antigen, it becomes activated and multiplies rapidly. Most of these activated B cells become plasma cells that produce and release thousands of antibodies per second.
Antibodies are Y-shaped proteins that work like guided missiles:
- Neutralisation: Antibodies bind to viruses and toxins, blocking them from entering cells
- Opsonisation: Antibodies coat pathogens, making them easier for phagocytes to engulf
- Clumping: Antibodies stick pathogens together, making them easier targets
- Complement activation: Antibodies trigger complement proteins to destroy pathogens
Some activated B cells become memory B cells that live for years or decades. If the same pathogen returns, memory B cells enable a much faster and stronger antibody response.
T Cells — The Cellular Killers
Directly destroying infected cells
While B cells fight pathogens in body fluids, T cells attack infected body cells directly. There are two main types:
Helper T cells coordinate the immune response by:
- Activating B cells to produce antibodies
- Activating cytotoxic T cells to kill infected cells
- Releasing chemical signals (cytokines) that recruit other immune cells
Cytotoxic T cells (killer T cells) directly destroy infected body cells by:
- Recognising infected cells that display foreign antigens on their surface
- Releasing toxic chemicals that punch holes in the infected cell
- Triggering the infected cell to self-destruct (apoptosis)
T cells are especially important for fighting viruses, which hide inside body cells where antibodies cannot reach them.
Primary and Secondary Responses
Why immunity gets stronger
The first time your body encounters a pathogen, it mounts a primary immune response:
- Takes 5-10 days to reach full strength
- Produces relatively few antibodies
- You may feel sick during this time
After the infection is cleared, memory B cells and memory T cells remain in the body. If the same pathogen is encountered again, the secondary immune response is dramatically different:
- Starts within 1-3 days
- Produces 100-1,000 times more antibodies
- The pathogen is usually eliminated before you feel sick
This is why you typically get diseases like chickenpox only once — your secondary response is so fast and strong that the virus is destroyed before it can cause symptoms. However, some pathogens (like influenza) mutate their antigens, allowing them to evade existing memory cells.
Common Misconceptions
"Antibodies are cells." No — antibodies are proteins produced by B cells (which are cells). Antibodies circulate in blood and body fluids, binding to specific antigens.
"The third line of defence works immediately like the first and second lines." No — the specific immune response takes 5-10 days to develop fully during a primary infection. This is why you feel sick for several days before recovering.
Australian Immunology Research
The Walter and Eliza Hall Institute (WEHI): Based in Melbourne, WEHI is Australia's oldest medical research institute and a world leader in immunology. WEHI scientists discovered how lymphocytes develop and function, and they continue to lead research on cancer immunotherapy, autoimmune diseases, and infectious disease immunity. Their work on apoptosis (programmed cell death) has transformed cancer treatment worldwide.
Immunology in Aboriginal health: Australian researchers study how the immune systems of Aboriginal and Torres Strait Islander people respond differently to certain infections. For example, Aboriginal Australians appear to have stronger innate immune responses but may have different patterns of adaptive immunity, which may explain why some infectious diseases present differently. This research helps develop more effective treatments and vaccines for Indigenous communities.
The Doherty Institute and T cells: During the COVID-19 pandemic, Melbourne's Doherty Institute conducted world-leading research on T cell responses to SARS-CoV-2. They found that T cells provided important protection even when antibody levels declined, informing vaccine strategies and public health policy in Australia and globally.
✍ Copy Into Your Books
▾Key Cells
- B cells: produce antibodies specific to antigens
- T cells: kill infected cells, coordinate response
- Memory cells: enable faster future responses
Antibodies
- Y-shaped proteins that bind to specific antigens
- Neutralise pathogens, mark them for destruction
- Produced by plasma cells (activated B cells)
Primary vs Secondary
- Primary: 5-10 days, slower, you feel sick
- Secondary: 1-3 days, 100-1000x stronger, usually no symptoms